Superjunction devices possess the advantage of significantly reduced Rdson for the same high breakdown voltage (BV) of a conventional MOSFET. The superjunction is comprised of a multi-layer, for example, a six-layer sequence of implant and epitaxy to form spaced P-columns which is used to balance the charge in the N type drift region epi which receives the columns. The same reticle is used repetitively on the six layers to generate the P-column.
The charge balance is critical with a small process window. Exceeding this window on the P-type side (that is, having an excessive P charge in the P columns) leads to the BV falling below the spec. Exceeding this window on the N-type side leads to high BV but can lead to ruggedness reduction.
Device ruggedness can be enhanced by structural modifications that force the current to flow through the P-column rather than outside it. Such structures are shown in copending application Ser. No. 60/417,212, filed Oct. 8, 2002 and assigned to the assignee of the present invention, and which is incorporated herein by reference. In that case, the top-most portion only of the P columns had a higher and unbalanced P concentration (charge) than the remainder of the columns, which have a balanced concentration against the surrounding N type body. This caused avalanche current at the top of the columns to be diverted from under the MOSFET source regions (the Rb′ region) and toward the axis of the column.